The present invention relates to a lubricating grease composition and preparation thereof, being more especially concerned with lubricating greases having improved consistency and homogeneity and low noise characteristics.
The lubricating grease types with which the invention is concerned comprise a major proportion of a lubricating base oil and a minor proportion of a simple soap or a complex soap thickener or of an organic thickener or inorganic clay thickener, such grease types include but are not limited to lithium simple and complex soap greases, calcium simple and complex soap greases, sodium simple and complex greases, aluminium simple and complex greases, barium simple and complex greases; as well as polyurea greases, organo-clay greases and bentonite clay/swelling-clay type thickeners.
It is an object of the present invention to provide a grease in which the thickener is of improved, e.g. smaller, more uniform, particle size. Such an improved structure will endow most grease types with e.g. improved consistency and useful life-time, and reduced oil separation. Although of general relevance, such an improved thickener is of especial advantage in lithium simple and complex soap greases.
As an example, lithium complex soap greases have found a major use as greases for bearings, particularly sealed bearings e.g. in automobiles and electrical appliances. Sealed bearing greases must meet numerous performance requirements including extended bearing life, high temperature performance, high dropping point; and defined requirements relating to oil separation, oxidation stability, fretting wear protection and low noise. However, although offering long service and high temperature benefits, such greases often suffer from generally higher noise characteristics than a lithium simple grease.
It is a further object of the present invention to provide a lubricating grease in general, but more especially a lithium complex grease, having reduced noise characteristics.
In accordance with one aspect of the present invention there is provided a method of preparing a lubricating grease composition having a major proportion of lubricating base oil and a minor proportion of a simple soap or a complex soap thickener system, or of an organic thickener system or inorganic clay thickener system; which method includes the steps of:
(a) agitating and heating the soap-forming or other grease-thickening constituent(s) with at least a portion-of the lubricating base oil in a closed reaction vessel over a temperature range and for a period of time sufficient to form a soap or other thickener system,
(b) cycling at least a portion of the contents of the reaction vessel, during at least the initial stage of the soap-forming, or other thickening, period, one or more times round a pressure-ventable closed processing loop comprising (i) the said agitating and heating in the reaction vessel and (ii) shearing the forming soap, or other thickener, in mechanical shearing means.
It is much preferred to conduct the cycling step (b) more than once during the said initial stage; and very preferably it is conducted over substantially the full soap-forming, or other thickening, period. Preferably, too, substantially all of the contents of the reaction vessel are cycled in at least one of the cycles of step (b); and, more preferably, in substantially all cycles.
The mechanical shearing is very preferably conducted with a shearing/milling/grinding gap of between 50 xcexcm and 2000 xcexcm, especially from 50 xcexcm to 500 xcexcm, more especially 100 xcexcm to 500 xcexcm. Suitably the mechanical shearing may be conducted employing one of the SUPRATON(copyright) machines e.g. the S400 model commercially available from Krupp Industrietechnik GmbH, Grevenbroich, Germany. Gaps between stator and rotor are set within the above ranges and, very preferably, reaction mix from the reactor is fed such that the dwell period of the material in the grinding gap is as extended as possible.
The following description relates to soaps and soap-forming, but is relevant to, and does not exclude, other types of thickening as referred to above.
In the reaction vessel (grease-making kettle) itself the soap-forming constituents are heated over a continuing temperature gradient and agitated (i.e. thoroughly mixed, for example by means of a double-motion agitator) both initially and upon each return from shearing. The combination of the steps (a) and (b) of the process of the invention permits the generation of an extremely large and active contact surface between reactants with slow evaporation of water. As a result crystallization occurs such that the soap structure formed has, among other properties, a small size and a high measure of noise-absorption.
The invention thus provides a means of greatly decreasing that portion of the noise-causing characteristics of a grease which are due to the internal soap crystal structure per se. It is known that externally introduced contaminants will increase the noise rating of a grease, for example, reaction components contaminated prior to or during manufacture. Therefore, it is highly preferred in the process of the present invention to employ components in as impurity-free form as possible, such as by filtering them in liquid form through break-proof filter systems. In this way noise level of the grease product is still further decreased. However, the essence of the present invention is a significant reduction in noise due to the internal soap crystal structure; and it is achieved by the controlled crystallisation which occurs during the saponification stage of the process of the invention.
A wide variety of lubricating base oils can be employed in the process and grease compositions of the present invention. Thus, the lubricating oil base can be any of the conventionally used mineral oils, synthetic hydrocarbon oils or synthetic ester oils, or mixtures thereof depending upon the particular grease being prepared. In general these lubricating oils will have a viscosity in the range of about 5 to about 400 cSt at 40xc2x0 C., although typical applications will require an oil having a viscosity ranging from about 10 to about 200 cSt at 40xc2x0 C. Mineral lubricating oil base stocks used in preparing the greases can be any conventionally refined base stocks derived from paraffinic, naphthenic and mixed base crudes. Synthetic lubricating oils that can be used include esters of glycols such as a C13 oxo acid diester of tetraethylene glycol, or complex esters such as one formed from 1 mole of sebacic acid and 2 moles of tetraethylene glycol and 2 moles of 2-ethylhexanoic acid. Other synthetic oils that can be used include synthetic hydrocarbons such as polyalphaolefins; alkyl benzenes, e.g., alkylate bottoms from the alkylation of benzene with tetrapropylene, or the copolymers of ethylene and propylene; silicone oils, e.g., ethyl phenyl polysiloxanes, methyl polysiloxanes, etc., polyglycol oils, e.g., those obtained by condensing butyl alcohol with propylene oxide; carbonate esters, e.g., the product of reacting C8 oxo alcohol with ethyl carbonate to form a half ester followed by reaction of the latter with tetraethylene glycol, etc. Other suitable synthetic oils include the polyphenyl ethers, e.g., those having from about 3 to 7 ether linkages and about 4 to 8 phenyl groups.
For lithium complex soap greases, preferred thickeners contain two, more preferably, three lithium components. The first may be a lithium soap of at least one, hydroxy fatty acid, preferably C12 to C29. The second may be selected from a lithium compound of (i) a C2 to C12 aliphatic or cycloaliphatic dicarboxylic acid (or C1 to C10, such as C1 to C4, alkyl ester thereof); or (ii) of a C3 to C24 hydroxy carboxylic acid (or C1 to C10, such as C1 to C4, alkyl ester thereof ) which has the hydroxy group separated from the carboxyl group by six or less carbon atoms; or a mixture thereof. The third component, which is very preferably present, is a lithium salt of boric acid.
Preferred hydroxy fatty acids include hydroxystearic, hydroxy-ricinoleic, hydroxybehenic and hydroxypalmitic. Especially preferred is 12-hydroxystearic acid. The second lithium compound is preferably a C6 to C10 aliphatic dicarboxylic acid, more preferably azelaic or sebacic acids, especially azelaic acid, or said ester of any of these. The C3 to C24 hydroxy-carboxylic acid is preferably lactic acid, salicylic acid or other hydroxy-benzoic acid, more preferably salicylic acid or a said ester of any of these. The amount of lithium soap complex thickeners is very preferably from 5 to 20 wt. %, based on grease. The weight ratio of hydroxy fatty acid to aliphatic dicarboxylic acid and/or hydroxy-carboxylic acid is preferably from 10:0.5 to 10:15, very preferably 10:1.5 to 10:6. The weight ratio of boric acid to the dicarboxylic and/or hydroxy carboxylic acid will preferably be from 1:5 to 1:20 very preferably 1:10 to 1:15.
In accordance with another aspect, the invention provides lubricating greases, both as broadly and more specifically defined herein, having a noise level of not above 3.5 units when measured by the SKF xe2x80x9cBEQUIEIxe2x80x9d test rig referred to herein. A level of 2.0 to 3.0 units is one achievable range.
In accordance with the invention, there is provided a lubricating grease composition comprising (i) a major amount by weight of a lubricating base oil; (ii) from 2 to 30 wt. %, preferably 2 to 20 wt. %, of a lithium complex soap thickener; and (iii) 0 to 10 wt. %, suitably 0 to 5 wt. %, of conventional additives, and having (a) a noise level below 3.5 units when measured as herein referred to.
Such a lithium complex grease will normally have a dropping point (ASTM D 566-87) of at least 270xc2x0 C., usually at least 2900xc2x0 C.